The present invention relates to a multistage centrifugal compressor, and more particularly, to a multistage centrifugal compressor including an annular suction passage upstream of a centrifugal impeller.
Japanese Unexamined Patent Application Publication No. 2006-152994 discloses a multistage centrifugal compressor provided with an annular suction passage for guiding the flow at the outlet of the return channel in the former stage to the blade inlet, the centrifugal impeller, the diffuser disposed downstream of the centrifugal impeller, and the return channel for guiding the fluid at the outlet of the diffuser to the next stage. The annular suction passage has each shape at the hub side and the shroud side connected with a smooth curve, and has the passage cross-section area of the annular suction passage at the eye portion (where the radius of the passage at the shroud becomes minimum) set to be larger than that of the blade inlet so as to prevent deceleration of the flow passing from the eye portion to the blade inlet.
In order to form the suction passage into the smooth shape, and to make the passage cross-section area of the eye portion larger than that of the blade inlet, the minimum radius of the suction passage at the hub side has to be reduced. The diameter of the rotary shaft has to be reduced to lower the critical speed of the rotary shaft system. The reduction in the critical speed may cause the problem of failing to increase the operation speed of the compressor.
The minimum radius of the passage at the hub side may be increased to prevent reduction in the critical speed. In the case where the suction passage is formed into the smooth shape, and the area of the eye portion is made larger than that of the blade inlet, the radius of the blade inlet is increased, and accordingly, the relative speed at the inlet is also increased to further bring the frictional loss against the impeller and the deceleration loss into the serious state. The efficiency of the compressor, thus, is deteriorated.